1. Field of the Invention
The present invention relates to a motor control circuit and a motor drive system using the same and, more specifically, relates to a motor control circuit used for devices such as video tape recoders, flexible disk drive devices (FDD), hard disk drive devices (HDD) and photo-magnetic disk drives which includes a Hall element as a rotating phase detection element or a rotating speed detection element for a motor and is designed so as to facilitate level adjustment of output signals generated from the Hall element even when the Hall element has some characteristic variation.
2. Description of Related Art
FIG. 3 shows a prior art three phase motor control circuit diagram.
The motor control circuit comprises three Hall elements 1,1,1, three differential amplifying circuits 2,2,2, an input amplifying circuit 3 including three switching circuits 5,5,5 (for avoiding complexly only one switching circuit 5 is illustrated in the drawing) and a motor drive circuit 4 for supplying drive currents to a motor 9 including three coils 8a, 8b and 8c. Further between the respective Hall elements 1 and the power source line +Vcc resistors (current limiting resistors) 6,6,6 for current limiting are connected and between the respective Hall elements 1 and the switching circuits 5 resistors (bias resistors) 7 for providing a bias voltage for the Hall elements 1 are connected, and these elements constitute a rotating phase detection circuit or a rotating speed detection circuit (hereinafter represented by the rotating phase detection circuit) for the motor 9.
The connecting conditions of the resistors 6, 7 to the backward Hall elements 1 in the drawing are the same as that to the front side Hall element 1. The respective phase detection circuits constituted by the respective Hall elements 1 and the resistors 6 and 7 are provided in parallel with respect to the power source line +Vcc. Further, when the switching circuit 5 has a large current carrying capacity, only one switching circuit which receives the currents from the respective Hall elements 1 is satisfactory. A detection circuit other than the above phase detection circuits constituted by three Hall elements connected in series between the resistors 6 and 7 may be used. In the both circuits the three Hall elements 1 operate in the same manner, hereinbelow the operation of one selected Hall element is pricipally explained.
Hall elements having four terminals e employed for the Hall elements 1, the Hall element 1 having two output terminals other than the connecting terminal for the current limiting resistor 6 and the connecting terminal for the bias resistor 7. When a magnetic field surrounding the Hall elements 1 varies in accordance with the rotation of the motor 9, a potential difference in response to the variation appears between the two output terminals of the Hall elements 1. The output signal generated from the Hall element 1 in response to the rotation of the motor 9 is a linear voltage signal having a sinusoidal waveform or the like. In order to generate such an output signal from the Hall element 1, it is necessary to supply beforehand a predetermined bias current I to the Hall element 1. The bias current I flows into the Hall element 1 from the power source line +Vcc via the current limiting resistor 6, when the switching ciruit 5 is rendered conductive upon receipt of a control signal A (see the drawing) which is generated in response to a power source turn-on signal or a motor start-up signal, and flows out to the ground line via the bias resistor 7 and the switching circuit 5. The value of the bias current I is primarily determined by the sum of the resistance of the current limiting resistor 6 and the bias resistor 7 and is adjusted thereby. Further, the voltage of the power source line +Vcc is divided by the current limiting resistor 6 and the bias resistor 7 and the bias voltage for the Hall element 1 is determined by the resistance value of the bias resistor 7 and is adjusted thereby. As a result, the Hall element I is provided with the bias current I and the bias voltage by the two resistors 6 and 7 for level adjustment use and is rendered into active condition. Thereby the detected signal V representing a motor rotating phase is outputted from the two output terminals to the differential amplifying circuit 2 in the input amplifying circuit 3.
The inverted terminal and the non-inverted terminal of the differential amplifying circuit 2 are respectively connected to the two output terminals of the Hall element 1. A resistor connected is to the input side of the differential amplifying circuit for determining the amplification rate and is omitted in the drawing. The differential amplifying circuit voltage-amplifies the signal v received at the inverted input terminal and the non-inverted input terminal and outputs a signal s corresponding to the output signal from the Hall element 1 to the motor drive circuit 4.
The motor drive circuit 4 comprises a control circuit 4d and output stage transistor circuits 4a, 4b and 4c for the motor drive currents. The control circuit 4d receives the three signals S from the input amplifying circuit 3, detects a motor rotating phase based upon these signals and drives the output stage transistor circuits 4a, 4b and 4c in response to the detected rotating phase. Through the control of the control circuit 4d, the output stage transistor circuit 4a sends out a drive current to the coil 8a, the output stage transistor circuit 4b sends out a drive current to the coil 8b and the output stage transistor circuit 4c sends out a drive current to the coil 8c. Accordingly, the motor 9 is controlled by the motor drive circuit 4 so as to rotate at an optimum rotating speed.
In the drawing, the circuit portion surrounded by a dotted line is a portion of the motor control circuit formed be in an IC. In the portion of the motor -control circuit formed in the IC, the differential amplifying circuit usually serves as a major portion of the input amplifying circuit. It is necessary to match the bias level for the operating point of the output signal from the Hall element with the input bias level under the static characteristic of the differential amplifying circuit.
A differential amplifying circuit of an NPN transistor having a high integration efficiency and having a large amplification rate with respect to the output signal from the Hall element is frequently used. In the differential amplifying circuit using an NPN transistor the input signal is usually applied to the base of the differential transistor, thereby, the bias level of the input signal is raised higher by 1Vf (the forward voltage drop of the PN junction) than the ground level. Therefore the output signal from the Hall element can not be sufficiently amplified unless the bias level of the signal at the Hall element side raises equal to or more than the bias level under the static characteristic of the input signal from the differential amplifying circuit.
For the above reason, the two resistors for level adjustment use are necessary at the Hall element side, one for the current adjustment use for determining the amplitude of the output signal and the other for the bias voltage setting use for determining the lower limit level of the output signal. The resistance values of these resistors are selected dependent upon the characteristic variation of the Hall element. Therefore the current limiting resistor is externally equipped to the power source side and the bias resistor is externally equipped to the ground side via the switching circuit. Accordingly, in the conventional three phase motor control circuit, the respective two resistors are externally equipped in addition to the respective Hall elements to thereby increase externally equipped components. Further, the voltage induced by the bias resistor varies dependent upon the drive current flowing through the Hall element, therefore it is necessary to select a proper combination of the three components in view of the characteristic of the Hall element and the resistance values of the two resistors.
As a result, there arises problems of a poor mounting efficiency of the three components during assemblage thereof and of a poor work efficiency thereof in the course of production of the motor drive devices including the motor control circuits formed in an IC, the Hall elements and the resistors. On the other hand, when the resistance values of the two resistors are fixed the work efficiency in the production of the devices is improved, however the selectable characteristic band of the Hall elements is narrowed dependent thereto and as a result, there arises a drawback that the component cost increases.